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List:       freebsd-net
Subject:    Notification about tentative IPv6 address from route socket
From:       Steffen Christgau <mail () s14u ! de>
Date:       2020-04-30 16:40:37
Message-ID: 4f3a314a-f938-476b-f75e-e495756a5488 () s14u ! de
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Hi everybody,

I'm quiet new in developing applications with support for FreeBSD, so
please excuse if that question has an obvious answer which I can't see
at the moment.

I'm developing a small daemon which should detect and handle changes of
IP addresses in order to open new sockets and close them when addresses
come and go. I implemented this so far by using PF_ROUTE sockets on
FreeBSD 12.1. This works quiet well and I'm able to see additions and
deletions of addresses.

For IPv6, however, I get notified about an address which is tentative (I
use ifconfig to add an link-local address manually) and I -
understandably - cannot bind a socket to that address. While I assume
that I can check for the address being tentative with ioctl(fd,
SIOCGIFAFLAG_IN6, ...), I'd like to ask the following:

1) Is there a way to get notified about the address being available for
usage (i.e. not tentative anymore, not duplicated) without _polling_ via
ioctl? My application is currently almost completely event driven and
integrating such a repeated timeout based detection of
tentative/duplicated addresses appears a little awkward. In addition
blocking until the address state changes is not really an option.

2) I know FreeBSD is not Linux, but on Linux with netlink sockets I get
notified about a) the address appearing (including flags) and b) some
time later the address being not tentative anymore (detectable via
flags). I wonder why the route socket on FreeBSD reports an address that
is currently hardly to use. On the other hand, I understand that
RTM_NEWADDR does exactly what is documented, i.e. to notify about an
"address being added to iface". Nevertheless, wouldn't it make sense to
tell an application that a change for an address takes place? I couldn't
observe such a behavior for IPv6 on FreeBSD. If there is currently no
really such notification, the kernel could emit a new message like
RTM_ADDRCHANGE or it may repeats the emission of RTM_NEWADDR (which
might confuse existing applications). Also the addition of an address
flags field to ifa_msghdr message might be beneficial. It would also
avoid additional ioctl calls and the boilerplate required around that.
Just thinking aloud... However, if there is already a notification-based
mechanism to get informed about a ready-to-use IPv6 address, could you
please provide me a pointer to that?

Looking forward for your responses.

Regards, Steffen
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